1. Field of the Invention
The present invention relates to a television tuner.
2. Description of the Related Art
A known television tuner will be described next with reference to FIG. 4. As shown in FIG. 4, an interstage double-tuned circuit 53 (hereinafter simply referred to as a “double-tuned circuit”) is provided between a high-frequency amplifier 51 and a mixer 52. A primary tuning circuit 53a includes a high-band tuning coil 54, a low-band tuning coil 55 connected in series to one end of the high-band tuning coil 54, and a varactor diode 56 connected in parallel to these tuning coils 54 and 55. An anode of the varactor diode 56 is connected to ground. A cathode of the varactor diode 56 is connected to the other end of the high-band tuning coil 54 via a direct-current blocking capacitor 57.
A secondary tuning circuit 53b also includes a high-band tuning coil 58, a low-band tuning coil 59 connected in series to one end of the high-band tuning coil 58, and a varactor diode 60 connected in parallel to these tuning coils 59 and 60. An anode of the varactor diode 60 is connected to ground. A cathode of the varactor diode 60 is connected to the other end of the high-band tuning coil 58 via a direct-current blocking capacitor 61.
The low-band tuning coils 55 and 59 are connected to one end of a coupling coil 62. The other end of the coupling coil 62 is connected to ground via a direct-current blocking capacitor 63.
A connection point between the high-band tuning coil 54 and the low-band tuning coil 55 is connected to a cathode of a switching diode 64. An anode of the switching diode 64 is connected to ground via a direct-current blocking capacitor 65. Similarly, a connection point between the high-band tuning coil 58 and the low-band tuning coil 59 is connected to a cathode of a switching diode 66. An anode of the switching diode 66 is connected to ground via a direct-current blocking capacitor 67.
The anode of the switching diode 64 is connected to a high-band switching terminal 69 via a resistor 68. Similarly, the anode of the switching diode 66 is connected to the high-band switching terminal 69 via a resistor 70.
Additionally, a connection point between the coupling coil 62 and the direct-current blocking capacitor 63 is connected to a low-band switching terminal 72 via a resistor 71. Furthermore, a tuning voltage is applied to the cathode of the varactor diode 56 from a tuning voltage terminal 73 via a resistor 74. Similarly, the tuning voltage is also applied to the cathode of the varactor diode 60 from the tuning voltage terminal 73 via a resistor 75.
A peaking circuit 76 is provided between the double-tuned circuit 53 and the mixer 52. A direct-current blocking capacitor 77 blocks a direct current flowing into the peaking circuit 76 from the double-tuned circuit 53. Also, a direct-current blocking capacitor 78 blocks a direct current flowing into the peaking circuit 76 from the mixer 52.
The peaking circuit 76 includes a peaking coil 79, a switching diode 80, and a direct-current blocking capacitor 81, all of which are connected in series. A cathode of the switching diode 80 is connected to the low-band switching terminal 72 via a resistor 82. An anode of the switching diode 80 is connected to the high-band switching terminal 69 via a resistor 83. The inductance of the peaking coil 79 and an input capacitance 84 of the mixer 52 together resonate with the low range frequency of a television signal in a high-band television signal.
In such a structure, when receiving a high-band VHF television signal, both switching diodes 64 and 66 conduct by applying a switching voltage to the high-band switching terminal 69 so that the double-tuned circuit 53 receives a high-band television signal. Since the switching voltage applied to the high-band switching terminal 69 is also applied to the anode of the switching diode 80, the switching diode 80 also conducts. Accordingly, a parallel resonant circuit is formed by the peaking coil 79 and the input capacitance 84 of the mixer 52.
The parallel resonant circuit formed by the peaking coil 79 and the input capacitance 84 of the mixer 52 increases the gain in the low range of the high-band television signal. Additionally, a tuning voltage applied to the tuning voltage terminal 73 causes the double-tuned circuit 53 to be tuned to a desired frequency.
When receiving a low-band VHF television signal, both switching diodes 64 and 66 do not conduct by applying a switching voltage to the low-band switching terminal 72 so that the double-tuned circuit 53 receives the low-band television signal. Since the switching voltage applied to the low-band switching terminal 72 is also applied to the cathode of the switching diode 80, the switching diode 80 also does not conduct. Accordingly, the peaking circuit 76 is disabled (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-275478, in particular, FIG. 2).
As described above, known television tuners increase the received frequency range by providing a switching diode in a tuning circuit in order to switch bands, and therefore, the structure of the tuning circuit becomes complicated. To increase the received frequency range using a tuning circuit without band switching, the tuning voltage must be significantly lowered. However, since the tuning voltage is output from a PLL (phase-locked loop) circuit, the low tuning voltage is not stable, which is a problem.